<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
       .bw{
           height: 300px;
           width: 200px;
           background: rgba(28, 204, 12, 0.9)
       }
    </style>
</head>
<body>
    <div class="bw">
        背景透明度
    </div>
</body>
</html>